AS Titrations?

Hi, I was able to do the 'simpler' titrations that we were given, but I keep doing this one but I don't get the correct answer (I don't think I do at least)... Could anyone explain where I went wrong/why? Thanks!

"A 1.575g sample of ethanedioic acid crystals, H2C2O4.nH20 was dissolved in water and made up 250cm^3. One mole of the acid reacts with two moles of NaOH. In a titration, 25.0cm^3 of this solution of acid reacted with exactly 15.6cm^3 of 0.16moldm^-3 NaOH. Calculate the value of n".

I realised that you just do a 'normal titration' and you either multiply both volumes by 10, or divide the g by 10.

So I did: 15.6cm^3/1000 = 0.0156dm^3
0.16 mol dm^-3*0.0156=0.002496 moles.

1 mole of acid : 2 moles of NaOH.

So 0.002496/2=0.001248moles

then you use the g/mol formula, so divide the 'g' by 1.575 by 10 to get 0.1575. So 0.1575/0.001248=126

And then the RFM of H2C2O4= 90. So 126-90=46. Then you divide by RFM of water (18) to get n. So 46/18 = 2.5. So n = 2.5? But loads of other people say it's like 50? :s

You have correctly calculated the moles in 25 ml, so to get the moles in 250 ml you have to multiply by 10 (there is ten times as much in 250ml as in 25ml)

You have correctly calculated the moles in 25 ml, so to get the moles in 250 ml you have to multiply by 10 (there is ten times as much in 250ml as in 25ml)

That is incorrect.

He found the moles in 25cm3, as that is what reacted with the NaOH (which you found the moles of by using c x v)

Because the moles worked out is in 25cm3, you divide the mass by 10 to find the mass of H2C2O4 in 25cm3. Then, work from there like OP has.

OP has made a mistake in his last line. 126-90 = 36, not 46. Therefore, 36/18 = 2.

n=2

"Please rate some other members before rating this member again." - I've rated you before? Maybe you really are 'that' helpful

Well surely dividing the amount of grams of substance by 10 is the same?

I found the moles for 25ml. (which is 1/10 of 250ml). So I find 1/10 of the original mass, which is 1.575, so is 0.1575?

Right, I misunderstood what you did. I meant that you have to multiply the MOLES of oxalic acid by 10 ...

and as you correctly say this is equivalent to dividing the mass of the original unknown by 10 (although it has to be said I've never seen anyone do it that way!)

Well you could either divide the mass by 10 or multiply the volume by 10 (i assume more people do this then?)

Feels funny to know that when I was 8, you were doing the questions that I would end up doing 9 years later.